In an effort to reduce the amount of pollution emissions from gas-powered turbines, governmental agencies have enacted numerous regulations requiring reductions in the amount of oxides of nitrogen (NOx) and carbon monoxide (CO). Lower combustion emissions can often be attributed to a more efficient air distribution control process, with specific regard to fuel injector location, airflow rates, and mixing effectiveness.
Early combustion systems utilized diffusion type nozzles, where fuel is mixed with air external to the fuel nozzle by diffusion, proximate the flame zone. Diffusion type nozzles historically produce relatively high emissions due to the fact that the fuel and air burn essentially upon interaction, without mixing, and stoichiometrically at high temperature to maintain adequate combustor stability and low combustion dynamics.
An alternate means of premixing fuel and air and obtaining lower emissions can occur by utilizing multiple combustion stages. In order to provide a combustor with multiple stages of combustion, the fuel and air, which mix and burn to form the hot combustion gases, must also be staged. By controlling the amount of fuel and air passing into the combustion system, available power as well as emissions can be controlled. Fuel can be staged through a series of valves within the fuel system or dedicated fuel circuits to specific fuel injectors. Air, however, can be more difficult to stage given the large quantity of air supplied by the engine compressor.
Of importance to the operation of a combustion system is also regulating the amount of compressed air supplied to the combustion system for mixing and reacting with fuel and as providing a source of cooling air. Therefore, it is necessary to carefully control the distribution of compressed air entering the combustion system. A number of modern day gas turbine combustion systems include a flow sleeve encompassing a combustion liner, where the flow sleeve can at least partially regulate the amount of air entering the combustion system. One such combustion system 100 is shown in FIGS. 1 and 2. The combustion system 100 has a flow sleeve 102 encompassing a combustion liner 104. Air for cooling of the combustion liner 104 and for use in the combustion process is enters a channel 106 through a plurality of holes 108 and an open flow sleeve aft end 110. Such an arrangement has little way of controlling the amount of cooling air entering the passageway 106.
Referring now to FIG. 3, an alternate prior art combustion system 300 for controlling the flow of compressed air to the passageway 326 between the flow sleeve 302 and combustion liner 304 is depicted. In such an arrangement, the sealing interface between the combustion liner 304 and flow sleeve 302 is accomplished by a piston ring 308. The piston ring 308 has a cross sectional area sized to provide the proper preload to ensure sealing. However, this proper preload requires a large radial area to implement. Such radial area requirements can create implementation problems in addition to flow blockage issues due to their mere size. As a result, the flow blockages that can occur increase the pressure drop taken across this air inlet region, adversely affecting the performance of the combustion system. In addition, the sealing system performance of a piston ring is directly tied to the roundness of the sealing interface.